Optical device for helmet visor comprising aspheric mirror

ABSTRACT

The invention relates to an optical device for a system for presenting collimated images including an off-axis spherical mirror. The device has an additional aspherical mirror whose surface forms, in the plane of symmetry of the unfolded optical system, a curve whose radius of curvature is variable. The surface provides for correction of the image of a pupil of an eye given by the spherical mirror and the pupil image is rectified on the optical axis. The surface may be a paraboloid, an ellipsoid and it may exhibit symmetry of revolution.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical device to correct the imageof the pupil of the eye given by a spherical concave mirror. Morespecifically, a device according to the invention can be used to observean image that is corrected of distortion due to a spherical orsubstantially spherical concave mirror that is tilted with respect tothe direction at which the eye observes this mirror.

2. Discussion of the Background

The invention can be applied to a helmet visor especially but notexclusively for the pilot of an armed aircraft or helicopter or for theoperator of a training simulator.

A helmet visor is an image-presenting device that is integrated into ahelmet. The visor enables the wearer of the helmet, for example thepilot of an aircraft in flight, to observe visual informationsimultaneously with the view of the landscape or of the pilot's cabin,which he perceives most usually through a protective visor.

The presentation of appropriate information, for example in the form ofsymbols, provides assistance in piloting and navigation. Thus, for armedvehicles, the presentation of a reticule provides assistance in theaiming of a weapon.

The information may also consist of an image of the landscape acquiredby sensors other than the eye of the helmet wearer such as infraredsensors or visible light intensifiers to complement or replace directviewing.

Inside the helmet, an image generator comprises an imager, for example acathode-ray tube screen or a liquid crystal screen on which an image isformed.

The helmet most usually has a relaying optic to convey this image up acombiner which presents the conveyed image in a state where it issuperimposed on the view of the landscape.

In order that the pilot may simultaneously observe the landscape whichis viewed directly at infinity and the image from the imager, the latteris also focused at infinity by a collimation optic.

When the combiner is formed by a simple semi-reflective flat plate, thecollimation of the image can be achieved by an optic placed between thecombiner and the imager. A prior art embodiment of this kind has themajor drawback of requiring a collimation optic that takes up far toomuch space in relation to the restricted field of view that is obtained.

To reduce the space requirement, a combiner with optical power has beenproposed. A combiner of this kind provides its user with both thecollimation of the image and the superimposition of the collimated imageon the view of the landscape.

The prior art has a very rich variety of devices comprising a combinerwith optical power. First of all, a concave spherical mirror achieves anaverage quality collimation of an image placed at a particular point inspace located on the axis of the mirror and at a distance from thismirror equal to half of its radius of curvature. By placing an imager atthis point, the eye located on the axis of the mirror receives rayscoming from the imager after they are reflected on the spherical mirror,these rays being parallel and leading to the perception, by the eye, ofa collimated image. If, furthermore, the mirror is semi-reflective, itenables the same eye to observe the landscape by transparency. However,in a device of this kind, the imager is on the axis of thesemi-transparent spherical mirror and it conceals the field of view ofthe user.

To clear the user's view, the spherical mirror may be tilted withrespect to the normal to his/her face, and the user's eye is no longeron the axis of the mirror. This tilting has the major drawback ofleading to a collimated image that is affected by optical aberrations,especially off-centring aberrations, excessively limiting the use ofsuch a device.

In order not to conceal the field of view of the user while at the sametime limiting aberrations, the prior art teaches us the use of aparabolic mirror instead of a spherical mirror. The imager is placed atthe focal point of the paraboloid described by the mirror and the eyeobserves the mirror along a parallel to the axis of revolution of theparaboloid.

The collimated image perceived by the eye has no spherical aberrationbut remains affected mainly by a coma with highly penalizing effects,the extent of which increases very rapidly with the field. Thus, theimager, while being off the axis of the field of view, remains ahindrance in the field of view.

One improvement consists of the exploitation of a double reflection onthe parabolic mirror with an intermediate plane mirror placed at thelevel of the user's forehead and called an onward reflection mirror. Thetwo reflections are located on either side of the axis of theparaboloid. They make it possible to obtain a collimated image that isfree of coma and whose other aberrations remain acceptable for a fieldof view that is still fairly restricted.

The desire to reduce the hindrance due to the onward reflection mirrorhas led to a development of the prior art. A device using a parabolicmirror and double reflection exhibiting asymmetry with respect to theaxis of this mirror has been described. While this device reduces thehindrance in the field of view, it increases the aberrations, especiallyastigmatism. The device described comprises lenses that are tilted toreduce astigmatism. It also comprises a field lens to compensate for thefield curvature and compensates for the distortion by a deformation ofthe image during its generation: the image is formed on the screen ofthe cathode-ray tube of the imager with a distortion that is the reverseof that which it is forced to undergo when crossing the optical device.

Furthermore, the initial idea of collimation by a spherical mirror hasundergone new developments. Thus, a device has been described with asemi-transparent spherical mirror having a tilted axis, comprising aprism to compensate for the inevitable aberrations induced.

The prism is placed on the path of the light rays between the imager andthe spherical mirror. The aberrations are minimized overall by adaptingthe tilt and aperture of the prism. And the astigmatism is corrected byan additional optical element that must be cylindrical.

This device is essentially penalized by a small field.

In parallel with this, devices have been made with spherical mirrorshaving no tilt in relation to the axis of view and with a shift of theimager.

A device of this kind has a semi-reflecting plane mirror placed betweenthe spherical mirror and the user's eye, at the focal point ofcollimation of the spherical mirror.

From the imager to the user's eye, a light ray follows an optical pathwhere, successively, it strikes the semi-reflecting plane mirror a firsttime, is reflected from this plane mirror towards the spherical mirror,and is then reflected on this spherical mirror and sent back to theplane mirror, it strikes the plane mirror a second time and goes throughit to meet the eye.

The collection of spherical and plane mirrors is transparent for raysemitted by the landscape.

This type of device presents a high quality collimated image.

However, this design which implies a compromise between reflection andtransmission by the plane mirror, has the drawback of sending to the eyeonly a small part of the initial light intensity and of thus tooseverely limiting the conditions of use of a helmet visor fitted outwith this device.

The transmission of the useful image to the eye can be improved byslightly tilting the spherical visor with respect to the axis of view ofthe user and by subjecting a plane mirror to an anti-reflectivetreatment that is selective as a function of the angle of incidence ofthe light rays.

With this geometry, the first and second angles of incidence of one andthe same light ray on the plane mirror have distinct angular valueswhereby the selective anti-reflective treatment, by circumventing thestandard compromise between the reflection and the transmission of a raythat penalizes the above device, helps the reflection of the initial rayjointly with the transmission of the already reflected ray.

This device has a fairly wide field of view but is affected byaberrations due to the tilting of the axis of the spherical mirror.Certain aberrations are corrected by a tilted field lens and byspherical lenses.

The astigmatism and distortion are not excessive since the tilt issmall, but are not corrected optically. Only a compensation of thedistortion by a deformation of the image generated on the cathode-raytube screen can be considered.

This device has improved luminosity, however the presence of the planemirror between the eye and the spherical mirror most usually integratedinto the visor of the helmet is a major drawback were it nor for thecomfort and security of the eye on the one hand and for the high cost ofits anti-reflective treatment on the other hand.

The problem is to make a device for presenting images for helmets with aspherical visor where there is no element interposed between the eye andthe visor and which presents a collimated image that is satisfactory forthe user, namely an image that is devoid of troublesome aberrations andhas a wide field of view greater than or equal to 40°.

The use of a spherical part of the visor as a collimation element leadsto major aberrations which must be corrected at least partially.

This is why the invention proposes an optical device for a system forpresenting collimated images to a user comprising an imager and asubstantially spherical off-axis concave mirror characterized in thatthe optical device comprises an optical axis and an aspherical concavemirror tilted on the optical axis, the intersection of the asphericalconcave mirror with the plane of incidence of the optical axis being acurve with a variable radius of curvature to correct the distortion ofthe image presented to the user, said distortion being due to thesubstantially spherical off-axis concave mirror.

The light rays coming from the centre of the imager form the centralfield of the imager. The optical axis of the device corresponds to thepath of the ray from the central field which passes through the centreof the user's pupil.

The optical axis is most usually a broken line. For example, if theimage is presented to the user straight in front of him, the part of theoptical axis located between the eye and the spherical mirror issupported by a first straight line normal to the centre of the user'spupil, the optical axis has a break at the intersection of this firststraight line with the spherical mirror, and the image that thespherical mirror gives of this first straight line supports the nextsegment of the optical axis.

The aspherical concave mirror placed between the spherical mirror andthe imager is tilted with respect to the optical axis. The surface ofthe aspherical concave mirror is preferably a second-order or quadraticsurface.

If the invention is presented without a folding mirror, it is alwayspossible, after the theoretical positioning of the various opticalelements of the invention, to add one or more plane mirrors whichintroduce no aberration but make it possible to meet the constraints ofspace requirement, for example so that the device is adapted to theprofile of the user's head. An optic presented without folding mirror iscalled an unfolded optic.

The plane of symmetry of the unfolded optic contains the normal to theentrance pupil of the user's eye and the centre of the spherical mirror.

The intersection of this plane with the aspherical concave mirror is aplane curve with a radius of curvature that is variable such as anon-degenerate conic. The surface of the aspherical concave mirror isused to correct the distortion of the image presented to the user thatis due to the off-axis spherical mirror.

BRIEF SUMMARY OF THE INVENTION

In one embodiment of the invention, the optical surface of the tiltedaspherical concave mirror is supported by a paraboloid. In anotherembodiment, the aspherical concave surface rests on an ellipsoid.

The surface of the aspherical concave mirror is preferably a part of asurface of revolution. In this case, it has the advantage of beingeasier to make.

The axis of revolution of the aspherical concave mirror is located inthe plane of symmetry of the unfolded optic.

The off-axis image of the pupil of the eye is the first pupil image ofthe device, it is tilted with respect to the optical axis. From thisfirst tilted pupil image, the aspherical mirror according to theinvention gives a second pupil image rectified on the optical axis.

In the case of a paraboloid of revolution, the axis of revolution of theaspherical concave mirror is substantially parallel to the normal to thefirst pupil image.

The device also comprises a power lens substantially centred and placedbetween the spherical mirror and the aspherical concave mirror.

When the surface of the aspherical concave mirror is described by aparaboloid, the power lens conjugates the image of the pupil of theuser's eye, given by the spherical mirror, in the vicinity of the focalpoint of conjugation of the paraboloid.

The ellipsoid has the advantage of having two focal points at finitedistance. A surface of this kind is less easy to machine than aparaboloid but it achieves better correction because the ellipsoidalmirror is completely stigmatic for its two focal points and providesefficient conjugation of the vicinities of the focal points.

The invention has the advantage of correcting the distortion of theimage presented to the user's eye for a wide instrument pupil, forexample with a diameter of at least 15 millimetres, and for a wide fieldtypically greater than 40°. The instrument pupil is the zone of space inwhich the user of the instrument has to place the pupil of his eye inorder to use the instrument.

This correction is particularly beneficial when a distortion cannoteasily be imposed at the imager. Indeed, the prior art teaches us thatin order to correct the distortion of the image given by an opticalassembly, it is necessary to introduce a reverse distortion at theimager by electronic correction. This is easily done when the imager hasa cathode-ray tube but this approach is not suited to an imager such asfor example a light intensifier which does not have the necessaryadjustments of the image.

The invention can be integrated into a helmet visor having a wideinstrument pupil and a wide field.

Other features and advantages of the invention shall appear from readingthe following detailed description of a particular embodiment made withreference to the following appended drawings in which the opticaldiagrams are shown unfolded, namely without a plane mirror.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 gives a schematic and partial view of an optical device with anoptical off-axis spherical combiner mirror,

FIG. 2 shows an unfolded device according to the invention,

FIG. 3 shows a device according to the invention with a parabolic mirrorand a relay optic,

FIG. 4 shows a device according to the invention with an ellipsoidalmirror and a relay optic,

FIG. 5 shows the distortion that is corrected by the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the user's eye 3 observes a spherical mirror 1. In theposition of observation, the pupil plane 2 contains the pupil 11 of theeye, which is generally located 3 millimetres behind the cornea 12 ofthe eye 3.

A straight line 5 passes through the centre of the pupil of the eye 3and is for example normal to the pupil plane 2. It is noted that,depending on its orientation with respect to the user's face, thestraight line 5 may correspond to the user's view straight ahead or elseto an upward or downward view, a view towards one side or a view towardsthe opposite side.

The spherical mirror 1 is placed in front of the user. Its concavity isturned towards the user, and the surface of this mirror is in thevicinity of its point of intersection 6 with the straight line 5.

The spherical mirror 1 is supported by a sphere S whose centre 4 doesnot belong to this straight line 5. The plane of FIG. 1 is a plane inspace that contains the centre of the supporting sphere of the sphericalmirror 1 and the straight line 5 passing through the centre of the pupilof the eye 3. It is the plane of incidence of the straight line 5 on thespherical mirror 1. Most usually, this plane is the same as the planepassing through the centre of the pupil 11 and parallel to thetheoretical plane of symmetry of the user's face.

The radius 7 of the sphere S which passes through the point ofintersection 6 is distant from the straight line 5 by an angle θ. Anon-zero value of this angle θ characterizes an off-axis use of thespherical mirror 1. The spherical mirror 1 itself is said to be“off-axis”.

Consider an optical ray 8 which is symmetrical with the straight line 5of the optical axis with respect to the described radius 7 of thesphere. As a first approximation, an image 9 placed on this optical ray8 at a distance equal to half of the radius of curvature of the sphereis perceived by the user's eye 3 as being collimated to first ordersince the light rays coming from the image 9 thus placed are reflectedby the spherical mirror 1 towards the eye 3 in the form of a beam ofsubstantially parallel rays. The image 9 may be plane or may exhibitfield curvature. The plane tangential to the image 9 is preferablyperpendicular to the optical ray 8.

However, the collimation by reflection on the spherical mirror is notperfect. It is affected by spherical aberration, coma, astigmatism,field curvature and distortion as well as off-centring opticalaberration due to the off-axis spherical mirror 1. Various opticalelements will be described in order to obtain, from a luminous imagegiven by an imager, the perception by the user's eye of a high qualitycollimated image.

The spherical mirror 1 may be semi-transparent. In this case, light rays10 coming from the environment external to the spherical mirror 1,namely striking the convex face of this mirror, are transmitted to theeye 3 by the spherical mirror 1. This spherical mirror 1 thenconstitutes a combiner which superimposes a collimated image on thedirect view of the environment.

The central field is defined as the beam of light rays coming from aparticular point of the image 9 which is the centre of this image. Aparticular light ray is considered. This light ray belongs to thecentral field and passes through the centre of the user's pupil. Thepath of this light ray is the optical axis of the device used. Theoptical axis is generally a broken line. The straight line 5 supports apart of the optical axis. Most usually, the image is presented straightahead of the user, the straight line 5 is then substantially normal tothe user's face, but the image may be for example presented at the topof the user's resting field of vision at infinity and the straight line5 is then oriented in the corresponding direction.

In FIG. 2, paths of light rays inside an embodiment of a deviceaccording to the invention are shown.

In this embodiment, the imager, which is not shown, comprises forexample a cathode-ray tube or a liquid crystal screen. The screen mayalso be embodied, for example, by a section of a bundle of opticalfibres or a slide or the screen of a light intensifier tube. An imagewhose surface is of any kind is displayed on the screen of the imagerrepresented by its tangent plane 20. For example, if the image is plane,it is contained in the tangent plane 20. Hereinafter, for simplicity,the plane 20 designates either the screen of the imager or its tangentplane without distinction. The path of the light rays from the screen ofthe imager up to the user's eye 3 are plotted for this embodiment of theinvention. The user's eye 3 is represented by its pupil which bears thesame reference to simplify the figures.

The device has an off-axis spherical mirror 1 and an aspherical concavemirror 21. It also has a power lens 22 located between these two mirrors1 and 21.

In this embodiment, the optical device has a diffractive lens 23 betweenthe power lens 22 and the spherical mirror 1.

The light rays coming from the screen 20 of the imager strike theaspherical concave mirror 21.

The light rays reflected by the aspherical concave mirror 21 go throughthe power lens 22 and the diffractive lens 23 and then strike theoff-axis spherical mirror 1 which collimates the image received by theuser's eye 3.

The path of the light rays is now observed in the other direction,namely going from the user's eye 3 and backtracking through thedifferent optical elements towards the screen of the display. The raysare reflected by the off-axis spherical mirror 1. The particular raywhich is the optical axis is also reflected on the spherical mirror 1.

In a first part 31 between the centre of the pupil of the eye 3 and thespherical mirror 1, the optical axis is supported by a straight line 31which, in FIG. 2, is horizontal. This optical ray is reflected by thespherical mirror 1 at a second part 32 of the optical axis.

The plane of incidence of the optical axis on the spherical mirror 1 isthe plane containing the first and second parts 31 and 32 of the opticalaxis. It is the plane P of FIG. 2.

In the second part 32 of the optical axis, a first pupil image 24 isobserved. It is the image of the pupil of the eye 3 given by theoff-axis spherical mirror 1. The normal 25 to the plane that istangential to this first pupil image 24 does not have the same directionas the section 32 of the corresponding optical axis. The first pupilimage 24 is tilted on the optical axis.

The power lens 22 is preferably centred on this second part 32 of theoptical axis. The power lens 22 is placed for example so that the firstpupil image 24 is on the path of the light rays between the sphericalmirror 1 and the power lens 22. The assembly of the power lens 22 andthe diffractive lens 23 form a power set used to correct the residualaberrations of the image. The power set enables the optical deviceaccording to the invention to have high image quality. This power set isan optical element close to the first pupil image 24. It has very littleeffect on the pupil image 24.

The aspherical concave mirror is placed in the vicinity of the secondpart 32 of the optical axis, so as to be on the path of the rays thatcome from the pupil of the eye 1—since here the description is writtenwhile backtracking along the real path of the light rays coming from thescreen of the imager—and reflect these rays towards the screen 20 of theimager.

The useful part of the aspherical mirror 21 has a tangent plane forwhich the normal 28 is not parallel to the second part 32 of the opticalaxis. The aspherical mirror 21 is said to be tilted with respect to thisaxis.

In the plane P of incidence of the optical axis on the spherical mirror1 which is defined by the first two parts of the optical axis 31 and 32,the aspherical mirror 21 has a variable radius of curvature.

The device according to the invention shown in FIG. 2 is free of anyfolding mirror, namely it is presented without any plane mirror. Indeed,plane mirrors do not modify the optical function. They do not introduceand do not correct any aberration but they enable the optical rays tocircumvent obstacles such as the user's head. They are not necessary tothe description of the invention.

The plane P is the plane of FIG. 2. It is also the plane of symmetry ofthe optic presented in this figure. The plane P of FIG. 2 is the planeof incidence of the optical axis on the aspherical concave mirror 21.The intersection of the optical surface of the aspherical mirror 21 withthis plane P is a plane curve which has a radius of curvature at each ofits points. On shifting along the curve, the value of the radius ofcurvature varies. The value of the radius of curvature is not constantand, depending on the direction of shift, this value increases or elsedecreases. The variation of the radius of curvature along this curve issaid to be monotonic. One example having a simple mathematicalexpression of such a curve is a part of a non-degenerate conic. Theconic is for example a parabola or an ellipse.

The shape of the concave surface of the aspherical mirror 21 is suchthat the entire device provides for a correction of the aberrations onthe image presented to the user which are due to the off-axis sphericalmirror.

The aspherical mirror 21 corrects the residual aberrations of the pupilimage given by the spherical mirror 1 of the pupil of the eye 3, it isplaced in the vicinity of the image of the screen 20 and therefore hasbut little effect on this image. The aspherical mirror 21 enables theoptical device according to the invention to have high pupil quality.The aspherical mirror 21 has little effect on the quality of the imagebut a great effect on the quality of the pupil.

A second-order or quadratic geometrical surface describes the opticalsurface of the aspherical mirror 21. A surface of this kind has theadvantage of being relatively easy to model since it can be expressed ina suitable reference frame in the form of a second-degree polynomial.

The geometrical surface supporting the optical surface of the asphericalmirror 21 is for example a paraboloid or an ellipsoid.

The geometrical surface is preferably a surface of revolution. It thenhas the advantage of being fairly easy to manufacture. For theparaboloid and ellipsoid examples, we may then refer to parabolic andellipsoidal mirrors respectively.

The axis of revolution is preferably in the plane P of FIG. 2 or planeof symmetry of the unfolded optic. The orientation of the axis ofrevolution is such that the aspherical mirror 21 compensates for theobserved tilt of the pupil image 24 that the spherical mirror 1introduces into the optical device.

In the embodiment of FIG. 2, the geometrical surface is a paraboloid ofrevolution whose intersection with the plane of FIG. 2 is a parabola 26.The axis of revolution is identical with the axis 27 of the parabola 26.The axis of revolution is preferably directed in the direction of thestraight line 25 normal to the tangent plane of the first pupil image 24given by the spherical mirror 1 of the pupil of the user's eye 3.

The axis 27 and the straight line 28 of the plane P of FIG. 2 which isnormal to the centre of the aspherical mirror 21 are not parallel. Theirorientations are substantially different. The aspherical mirror 21corresponds to an off-centred part of a parabolic mirror since theuseful surface of the paraboloid is not in the immediate vicinity of thevertex of the parabola 26.

The aperture around the axis 28 is sufficient to optimize the gap thatis left available to place, for example, onward reflection mirrorsbetween the aspherical mirror 21 and the lens 22. And the angle ofincidence of the optical axis on the aspherical mirror 21 also makes itpossible to limit the useful surface and thus to preserve high imagequality throughout the surface. The angle of incidence is preferablyclose to 45°. In this embodiment, the useful surface area of the mirror21 is estimated for example by a diameter of about 45 millimetres.

In FIG. 3, an optical device according to the invention has a relayoptic 29 to distance the screen 20 of the imager from the asphericalmirror 21. This distancing is generally made necessary to meetconstraints of space requirement. It makes it possible for example, fora helmet visor, to place the entire imager, which may be a cathode-raytube, in a satisfactory position in the available volume of the helmet.

FIG. 3 shows the third part 33 of the optical axis which corresponds tothe reflection of the second part 32 of this same optical axis on theaspherical mirror 21. The relay optic 29 is placed between theaspherical concave mirror 21 and the screen 20 of the imager. It issubstantially aligned with the third part 33 of the optical axis. Thisessentially centred relay optic is simple to make.

A second pupil image 30 can be observed on the part 33 of the opticalaxis. This image is located between the aspherical mirror 21 and thescreen of the imager 20. This second pupil image 30 is seen by theaspherical mirror 21 through a set 35 of lenses belonging to the relayoptic 29.

The paraboloid of revolution has two focal points, one at infinity andthe other at a finite distance from its vertex. The aspherical mirror 21is preferably placed so that the pupil images 24, 30 are perceived bythe aspherical mirror 21 respectively through the power set 22, 23 andthe lenses 35 as being located at the level of its focal points. Thefirst pupil image 24 corresponds to the focal point at infinity. It issaid that the first image 24 of the pupil of the user's eye given by thespherical mirror 1 is seen by the aspherical mirror 21 through the powerset 22, 23 in a conjugate manner.

More specifically, the pupil image 24 is not quite placed optically atinfinity with respect to the parabolic mirror 21 since this would leadto an excessively large size for the power lens 22. The embodimentdescribed here by way of an example corresponds to sufficient opticaldistancing between the aspherical mirror 21 and the first pupil image24.

Furthermore, the magnification between the two pupil images 30 and 24preferably has a value close to one. The pupil conjugation practicallyequal to one has the advantage of reducing the space requirement of theoptical device. It enables the size of the optics to be minimized allalong the optical path. This reduction of space requirement isadvantageous for the weight of the device and for its cost.

The pupil image 30 has a tangent plane that is substantially normal tothe local optical axis 33. This is a correction made by the asphericalmirror 21. Indeed, the first image 24 of the pupil of the eye formed bythe spherical mirror 1 is tilted with respect to the local optical axis32 and corresponds to the aberrations induced by this mirror 1. Thesecond pupil image 30 is rectified with respect to the optical axis 33by the aspherical mirror 21.

The surface of the aspherical mirror 21 is such that this mirror 21rectifies the pupil image with respect to the optical axis. Thisrectification enables compensation for the distortion introduced by theoff-axis spherical mirror 1. This rectification of the pupil imagecorresponds to the correction of the pupil spherical aberration. Itreduces the distortion of the image observed by the user of a deviceaccording to the invention.

FIG. 4 shows the (folded) optical diagram of a device according to theinvention with a relay optic. This embodiment of the invention isdistinguished from the one described by means of FIGS. 2 and 3 since theaspherical mirror 21 here has a geometrical surface whose intersectionwith the plane P of symmetry of the unfolded optic, or plane ofincidence of the optical axis, is supported by an ellipse 46.

A power set comprising at least one power lens 42 and one diffractivelens 43 is placed between the first pupil image 24 and the asphericalmirror 21. In this embodiment, as in the one shown in the previousfigure, a relay optic 44 distances the plane 20 tangential to the imagerfrom the aspherical mirror 21. A second pupil image 40 is observed. Thisimage is seen by the aspherical mirror 21 through a set of lenses 45.

The surface of the mirror 21 is a part of an ellipsoid. Most usually,this surface has a symmetry of revolution. The axis of revolution 47 ispreferably located in the plane P of incidence of the optical axis.

The axis of revolution of the ellipsoid identical to one of theprincipal axes of the ellipse 46 defines an ellipsoidal mirror.

The axis of revolution 47 is such that the aspherical mirror 21 convertsthe first pupil image 24 tilted on the optical axis 32 into a secondpupil image 40 substantially perpendicular to the optical axis 41 onwhich it is located.

The aspherical mirror 21 sees the first pupil image 24 through the powerset 42, 43 and it sees the second pupil image 40 through the group oflenses 45.

The ellipsoid of revolution has two focal points at finite distance. Thefocal points of the ellipsoidal mirror 21 correspond preferably to thepositions around which the pupil images 24, 40 are seen by this mirror.The pupil images 24, 40 are said to be conjugated by the asphericalmirror 21.

The ellipsoidal mirror is completely stigmatic. It has the advantage ofproviding for good conjugation of surfaces placed in the vicinity of itsfocal points. It is more difficult to manufacture than a mirror thatfollows the shape of a paraboloid. It is therefore more costly, but adevice according to the invention comprising an ellipsoidal mirror is onthe whole more efficient.

The mirror 1 which hitherto has been most often described as a sphericalmirror may equally well be a concave mirror with a shape close to thatof the sphere which also induces a distortion of the image seen by theuser corresponding to a tilting of the pupil image with respect to theoptical axis. The invention enables the correction of the distortion dueto a substantially spherical concave mirror.

The substantially spherical off-axis concave mirror 1 may besemi-transparent. In this case, the light rays emitted by the landscapeof the environment in the field of view of the user are transmitted bythis mirror and are received by the pupil of the eye simultaneously withthe rays reflected by this same mirror and described earlier. Thesemi-transparent mirror is a combiner. It is therefore a substantiallyspherical concave combiner used off-axis.

This combiner preferably forms part of a visor to protect the user'seyes and even his face.

A visor according to the invention has at least one substantiallyspherical concave off-axis reflective part.

In the position of use, the visor is folded down so that the partcorresponding to the substantially spherical concave mirror 1 is placedin front of the user's eye. The entire device for presenting collimatedimages can be integrated into a helmet, for example for an aircraft orhelicopter pilot and enables the making of a helmet visor. The devicemay present only substantially collimated images since, in manypractical cases, it is enough to present images focused several metresin front of the user.

The distortion of an image exhibiting a grid pattern leads to thedeformation of the grid pattern. The off-centring distortion of thesecond kind which corresponds to the pupil spherical aberration inducedby the reflection on the substantially spherical off-axis concave mirror1 is represented in FIG. 5. This distortion gives the impression thatthe grid pattern is being seen in perspective.

The images presented to the user, and whose distortion inherent in theoff-axis substantially spherical concave visor is corrected, areparticularly advantageous for a helmet visor since they comply with thereal dimensions of the objects shown. This being useful when the visorpresents an image superimposed on the direct view and is even more sowhen, for the user, the image presented substitutes for the direct viewfor example in the event of poor visibility or in the event of a precisesimulation of the environment. The correction of this distortion has theadvantage of enabling the user to make a good assessment of distances onthe image that he observes, and of enabling him for example to pilot bynight without positioning errors.

What is claimed is:
 1. An optical device for presenting collimatedimages through a pupil of an eye of a user, comprising: an imager; asubstantially spherical off-axis concave mirror, in which a path of aray passing through a center of the pupil of the eye and a center of theimager forms intersecting optical axes, said substantially sphericalconcave mirror being tilted with respect to a first axis of theintersecting optical axes between said center of the pupil and saidsubstantially spherical concave mirror; and an aspherical concave mirrortilted on a second axis of the intersecting optical axes correspondingto a reflection path of the first axis, wherein an intersection of theaspherical concave mirror with a plane of incidence of the secondoptical axis is a curve with a variable radius of curvature so as tocorrect a distortion of an image presented to the user, said distortionbeing due to the substantially spherical concave mirror.
 2. The deviceaccording to claim 1, wherein along the curve, the variation of theradius of curvature is monotonic.
 3. The device according to claim 2,wherein the curve is a parabola or an ellipse.
 4. The device accordingto claim 1, wherein an optical surface of the aspherical mirror issupported by a paraboloid.
 5. The device according to claim 4, whereinthe optical surface of the aspherical mirror is supported by aparaboloid of revolution.
 6. The device according to claim 1, wherein anoptical surface of the aspherical mirror is supported by an ellipsoid.7. The device according to claim 6, wherein the optical surface of theaspherical mirror is supported by an ellipsoid of revolution.
 8. Thedevice according to claim 5, wherein an axis of revolution of a surfaceof revolution is substantially parallel to a normal to a planetangential to a first pupil image that the substantially sphericalconcave mirror gives of the pupil of the eye.
 9. The device according toclaim 5, further comprising: a power set placed between thesubstantially spherical concave mirror and the aspherical mirror,wherein an image of the pupil of the eye given by the substantiallyspherical concave mirror is seen by the aspherical concave mirrorthrough the power set in a conjugated mode.
 10. The device according toclaim 5, wherein the surface of the aspherical mirror is a part of aparaboloid and the aspherical mirror sees, substantially at infinity, afirst pupil image that the substantially spherical concave mirror formsof the pupil of the eye.
 11. The device according to claim 7, whereinwhen the surface of the aspherical mirror rests on the ellipsoid, theaspherical mirror sees, in the vicinity of one of its focal points, afirst pupil image that the substantially spherical concave mirror formsof the pupil of the eye.
 12. The device according to claim 1, whereinthe substantially spherical concave mirror is semi-transparent.
 13. Thedevice according to claim 1, wherein the optical device is a helmetvisor.
 14. The device according to claim 2, wherein an optical surfaceof the aspherical mirror is supported by a paraboloid.
 15. The deviceaccording to claim 14, wherein the optical surface of the asphericalmirror is supported by a paraboloid of revolution.
 16. The deviceaccording to claim 2, wherein an optical surface of the asphericalmirror is supported by an ellipsoid.
 17. The device according to claim16, wherein the optical surface of the aspherical mirror is supported byan ellipsoid of revolution.
 18. The device according to claim 15,wherein an axis of revolution of a surface of revolution issubstantially parallel to a normal to a plane tangential to a firstpupil image that substantially spherical concave mirror gives of thepupil of the eye.
 19. The device according to claim 17, furthercomprising: a power set placed between the substantially sphericalconcave mirror and the aspherical mirror, wherein an image of the pupilof the user's eye given by the substantially spherical mirror is seen bythe spherical concave mirror through the power set in a conjugated mode.20. The device according to claim 8, wherein the surface of theaspherical mirror is a part of a paraboloid and the aspherical mirrorsees, substantially at infinity, a first pupil image that thesubstantially spherical concave mirror forms of the pupil of the eye.21. The device according to claim 8, wherein when the surface of theaspherical mirror rests on the ellipsoid, the aspherical mirror sees, inthe vicinity of one of its focal points, a first pupil image that thesubstantially spherical concave mirror forms of the pupil of the eye.